<18.5 Normal
18.5–24.9 Overweight
25–29.9 Obese
>30
WHO BMI Classification
| Category | BMI Range | Health Risk |
|---|---|---|
| Severely Underweight | < 16.0 | Very High |
| Underweight | 16.0 – 18.4 | High |
| Normal Weight ✅ | 18.5 – 24.9 | Low |
| Overweight | 25.0 – 29.9 | Moderate |
| Obese Class I | 30.0 – 34.9 | High |
| Obese Class II | 35.0 – 39.9 | Very High |
| Obese Class III | ≥ 40.0 | Extremely High |
Frequently Asked Questions
📖 How to Use
- Select metric or imperial units.
- Enter your height and weight.
- Click Calculate BMI.
- Review your BMI score and weight category explanation.
BMI Calculator — Understanding Body Mass Index Beyond the Number
Body Mass Index (BMI) is a numerical value derived from a person's weight and height that provides a population-level screening tool for weight categories — underweight, normal weight, overweight, and obesity. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, BMI was originally a statistical tool for studying populations, not individuals. It became a clinical shorthand in the 1970s and remains the most widely used weight-assessment metric today despite significant limitations.
Understanding what your BMI number means — and, equally important, what it doesn't mean — is essential for interpreting the result correctly and making informed decisions about health.
The BMI Formula
BMI is calculated using one of two equivalent formulas depending on the unit system:
Imperial: BMI = [weight (lb) ÷ height² (in²)] × 703
Worked Example: A person weighing 75 kg who is 1.75 m tall:
BMI = 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.49 → Normal weight
WHO BMI Classification Table
| BMI Range | Category | Health Risk |
|---|---|---|
| <18.5 | Underweight | Increased risk (nutritional deficiency, immune function) |
| 18.5–24.9 | Normal weight | Lowest risk for weight-related conditions |
| 25.0–29.9 | Overweight | Increased risk for cardiovascular disease, T2DM |
| 30.0–34.9 | Obesity Class I | High risk |
| 35.0–39.9 | Obesity Class II | Very high risk |
| ≥40.0 | Obesity Class III | Extremely high risk |
Note: Asian populations have a higher disease risk at lower BMI values. Many Asian health guidelines use 23 as the "overweight" threshold and 27.5 as the "obesity" threshold — approximately 2 units lower than WHO global standards.
Critical Limitations of BMI
BMI has important limitations that every user should understand before acting on the result:
- Does not measure body fat — BMI measures weight relative to height, not fat percentage. A professional athlete with high muscle mass (which is denser than fat) may register as "overweight" or even "obese" by BMI despite having very low body fat
- Ignores fat distribution — visceral fat (around organs) is far more harmful than subcutaneous fat (under skin). Two people with identical BMIs can have very different metabolic risk profiles based on where they carry their weight
- Not valid for children — child BMI uses age- and sex-specific percentile charts, not the adult ranges above
- Not valid during pregnancy — weight gain during pregnancy is expected and healthy; standard BMI interpretations do not apply
- Ethnic variation — risk thresholds differ significantly across ethnic groups
Frequently Asked Questions
Is a BMI of 25 automatically unhealthy?
Not necessarily. BMI 25 sits at the boundary of "overweight" by WHO standards but is a weak predictor of individual health outcomes. Many people at BMI 25–27 have perfectly healthy metabolic markers (blood pressure, blood glucose, cholesterol). Use BMI as one data point among several — waist circumference, blood tests, and fitness level matter more for individual assessment.
What is a healthy BMI for Pakistani adults?
Pakistani and South Asian guidelines typically use a threshold of BMI ≥23 for "overweight" and ≥27.5 for "obesity" due to evidence that South Asians accumulate more visceral fat at lower BMI values than Western populations. These tighter thresholds better predict metabolic risk in Pakistani and Indian populations.
My BMI is in the "normal" range but I feel unhealthy — why?
BMI does not capture fitness level, muscle mass, bone density, diet quality, stress, sleep, or dozens of other health factors. You can have a normal BMI and still be metabolically unhealthy if you have low muscle mass, high visceral fat (common in sedentary "thin" individuals), poor diet, or chronic stress. This is sometimes called "normal weight obesity" or TOFI (Thin Outside Fat Inside).
Should I use BMI or waist-to-height ratio?
Waist-to-height ratio (WHtR) is increasingly recommended as a better predictor of cardiovascular and metabolic risk than BMI because it directly reflects abdominal fat. A WHtR above 0.5 (waist circumference more than half your height) is associated with increased risk across all ethnic groups. Ideally use both alongside a doctor's assessment.
Can children use this BMI calculator?
No — this calculator uses adult WHO categories and is not valid for anyone under 18. Child BMI is assessed using age- and sex-specific growth chart percentiles from the CDC or WHO child growth standards. For children, consult a paediatrician for proper assessment.